Numerous clinical observations indicate that in certain patients alloantibodies probably contribute to rejection. Although some investigators have attributed a causal relationship to this association, there are still insufficient data to evaluate critically the mechanisms of such antibody-mediated acute allograft injury. Presently, the availability of inbred "knock out" mice, monoclonal antibodies to critical mediators, and soluble recombinant receptors provide the means to establish a causative role of antibody in the in vivo pathogenesis of acute graft rejection. Recent in vitro studies have demonstrated that antibodies and complement can initiate dynamic interactions among endothelial cells, macrophages and platelets that are mediated by adhesion molecules and their ligands. The potential role of these receptors and their ligands has not been examined in acute alloantibody-mediated rejection. The hypothesis to be tested in this proposal is that alloantibody and complement mediate acute cardiac rejection by activating endothelial cells, macrophages and platelets through ,expression of P-selectin and von Willebrand factor. The proposed studies are based upon our extensive preliminary data from cardiac allograft models in complement deficient rats as well as immunoglobulin knock out (IgKO), C4, C3 and CR2 KO mice. The specific aims of this project will test our hypothesis by demonstrating that: 1. Passive transfer of complement-activating classes and subclasses of mAbs specific for donor MHC antigens can reconstitute cardiac allograft rejection in IgKO mice and cause the upregulation of P- selectin, von Willebrand factor, and MCP-l in endothelial cells. 2. Passive transfer of non-complement-fixing subclasses of mAbs specific for donor MHC antigens will modulate in a dose-dependent manner cardiac allograft rejection induced in IgKO mice by complement-activating subclasses of mAbs with the same specificity. 3. Activation of endothelial cells, B-cells, platelets and macrophages by antibody mediated acute rejection is inhibited in C4, C3 and CR2 KO mice resulting in prolonged cardiac allograft survival. 4. P-selectin augments activation of endothelial cells, platelets and macrophages in allograft rejection.